You will have to remap capslock system-wide to accomplish this. You can remap capslock to any other modifier key (shift, command, option, control) using System preferences (Keyboard > Keyboard > Modifier Keys). Sublime cannot map commands to a single modifier key though (it would have to be a modifier + some other key).

Your best option is to use KeyRemap4Macbook to map caps lock to a function key (e.g. F5) and change your sublime user keybindings to map autocomplete to the function key.